Astronomy Picture of the Day

Explanation:

雙倍的煩惱，雙倍的麻煩；火焰燃燒，鍋泡沸騰……或許馬克白應該請教一下女巫首星雲。 這個形狀帶著強烈暗示性的反射星雲，與獵戶座的亮星參宿七有緊密的關聯。 正式名稱為IC 2118的女巫首星雲，主要是靠反射參宿七的星光來發光。 參宿七則座落在影像的右緣外頭，約一張照片寬度的地方。 在此星雲裡反射星光的，主要是其中的塵埃微粒。 此星雲的泛藍色澤，除了因反射參宿七的藍色星光之外，還因為塵埃微粒反射藍光的效率高於紅色。 相同的物理過程，也造就了地球的白晝的藍天，只不過地球的散射體是氮和氧分子。 此星雲距離我們約1,000光年遠。

Double, double toil and trouble; Fire burn, and cauldron bubble ... Maybe Macbeth should have consulted the Witch Head Nebula. This suggestively shaped reflection nebula is associated with the bright star Rigel in the constellation Orion. More formally known as IC 2118, the Witch Head Nebula glows primarily by light reflected from Rigel. Rigel is located about one photo-width off the image to the right. Fine dust in the nebula reflects the light. The blue color is caused not only by Rigel's blue color but because the dust grains reflect blue light more efficiently than red. The same physical process causes Earth's daytime sky to appear blue, although the scatterers here are molecules of nitrogen and oxygen. The nebula lies about 1000 light-years away.